Kazar, once you use the OGR Index to find the issue that a particular article is in, you can go to the Digital Library to download the issue. Digital Subs do not need to purchase anything extra. As a digital subscriber, you have access to every issue of OGR, all 300+ of them, starting with Run1, from June 1969.
That BUY option is there for non-digital subs who may want just that one issue.